We use the graph API from Microsoft to get the details about site, drives, files, folder, etc.
The graph API works as expected and returns the results for those queries.
However, If I try to get the permissions using /sites/{site_id}/permissions endpoint I get:
{
"@odata.context": "https://graph.microsoft.com/v1.0/$metadata#sites('{site_id}')/permissions",
"value": []
}

Sites.FullControl.Allpermissions? Check this documentation: learn.microsoft.com/en-us/graph/api/…. Let me know if it works for you.Sites.FullControl.All?